Carboxylic acids are important chemicals of commerce. They appear as desired or contaminating constituents of a wide range of aqueous and organic process streams. Historically, they were produced from animal fat or vegetable oil sources or from petroleum sources in substantially nonaqueous systems. More recently they have been identified among the most attractive products for manufacture from biomass (e.g. corn starch) by fermentation. In these more advanced processes, the carboxylic acid is generated as a dilute solution in an aqueous fermentation broth. Acetic acid is recovered commercially from dilute aqueous solutions by extraction with solvents such as isopropyl acetate, other esters, or ethers. Aqueous solutions are created during the manufacture of adipic acid. Citric acid is recovered from fermentation broths commercially by solvent extraction with high-molecular-weight tertiary amines (e.g. tridecylamine) in a diluent composed of a hydrocarbon (e.g. kerosene) and an alcohol (e.g. n-decanol). Citric acid commands a substantial market, which is increasing as detergent manufacturers switch to citric acid as "builder". Lactic acid (raw material for biodegradable plastics), succinic acid, malic acid, fumaric acid, and other carboxylic acids which may be manufactured on a large scale by fermentation of biomass are creating considerable interest in solvent extraction as a means of recovery. Carboxylic acids are also stable oxidation products and frequently appear as by-products or contaminants in aqueous and organic waste streams.
Biomass source streams and the like from which carboxylic acids are to be recovered are typically dilute aqueous solutions. Solvent extraction is one widely studied method for recovering carboxylic acids from such streams. This forms an organic solvented solution of the acids. A cost-effective solvent-extraction process for isolating carboxylic acids requires an efficient method for recovering the acids from the organic phase. A sufficiently volatile carboxylic acid can be recovered from an organic phase by distillation, as is done commercially for acetic acid. However, the dicarboxylic and hydroxycarboxylic acids have very low volatilities and cannot be recovered in this way.
Prior workers have suggested a variety of processes for recovering low-volatility carboxylic acids following extraction. A. Baniel et al., in U.S. Pat. No. 4,275,234, describe recovering citric acid from an organic extract by back-extraction into water following a change (usually an increase) in temperature. Evaporation and crystallization are still required to isolate the product acid. Baniel et al. also suggest back-extraction into water following a change in composition of the organic extract, e.g. by distilling a component out of the extract and recycling that component back to the extraction zone. Evaporation and crystallization of the back extract are again required to isolate the product acid. A similar process was set out by J. Tamada and C.J. King in a paper presented at the International Solvent Extraction Conference, Moscow, USSR, July 1988.
W. P. Ratchford et al., in U.S. Pat. No. 2,539,472, describe the extraction of water-soluble carboxylic acids from fermentation broths. R. D. Chapman et al., in U.S. Pat. No. 3,023,238, describe recovery of a carboxylic acid from an aqueous solution by evaporative crystallization. Solvent extraction of a mixture of carboxylic acids and fractional extraction to isolate the individual acids are described in Danly et al., U.S. Pat. No. 3,329,712. K. Konno, in U.S. Pat. No. 3,786,096, describes solvent extraction of adipic acid from an aqueous solution with cyclohexanone and/or cyclohexanol, but not the recovery of the acid from the extract. V. P. Kuceski, in U.S. Pat. No. 3,810,937, describes schemes for recovering adipic acid from solvent solutions that include evaporation of the solvent, esterification of the acid, and back extraction of the ester into water. Recovery and fractionation of carboxylic acids by codistillation with a suitable alkylbenzene is described by B. Baker, in U.S. Pat. No. 4,191,616.
Three references which concern changes in solubility of carboxylic acids in organic liquids with changes in water content of the organic phase are as follows: Forbes, G.S. and Coolidge, A.S., J. Am. Chem. Soc., 41(2),150-67, (1919) found increasing acid solubility as the water concentration in an organic solvent increased. The system they worked with was succinic acid--diethyl ether--water. They noted the change, but give no indication of applying it to the process which is this invention. Sato, T., Watanabe, H., and Nakamura, H., Bunseki Kaoaku, 34,557-63, (1984) reported that in the extraction of lactic acid, succinic acid, and citric acid into a mixed solvent of xylene and trioctylamine, the amount of water in the organic phase decreased with an increase in the acid concentration in the organic phase. Lipovskii, A.A. and Kuzina, M.G., Radiokhimiya, 10(2), 175-81, (1968) studied the extraction of oxalic acid into solutions of trioctylamine (TOA) in chloroform and TOA in benzene. The solid TOA-bioxalate salt with TOA:H.sub.2 C.sub.2 O.sub.4 ratios of 2:1 and 1:1 did not dissolve into anhydrous chloroform or anhydrous benzene. Water was added to the organic phase in order to get the salt to dissolve. However, the salt with TOA:H.sub.2 C.sub.2 O.sub.4 ratios of 1:1 and 1:2 did dissolve in anhydrous chloroform.